U.S. Pat. No. 4,185,883 (Chown et al.) discloses an optical fiber coupling element that includes a glass sleeve secured to a length of optical fiber. The optical fiber is placed in the glass sleeve, and the sleeve is heated so that it collapses around the fiber to hold the fiber in place.
U.S. Pat. App. No. 20020110332 (Clarkin et al.) discloses a ferrule presenting a preferentially “softenable” substance within the bore for reducing fusion temperatures and thus reducing distortion of the ferrule and element fused within.
U.S. Pat. No. 6,282,349 (Griffin) discloses a fiber fused in a ferrule.
It is often desirable to employ a ferrule in joining optical components together or when incorporating optical components into other devices. A ferrule is a piece of material (glass, ceramic, polymer, metal or composite) having one or more holes into which components such as optical fibers or capillaries may be inserted, offering added support to the fiber/capillary as well as increased structural strength and facilitate alignment of the fiber/capillary with another, similarly terminated component.
In most cases, adhesives secure components carried within ferrules but adhesives can be problematic in many applications due to the difference in chemical and physical properties of the adhesives with respect to the fiber and ferrule. Adhesive terminated fibers in ferrules are not truly hermetic and are labile to elevated temperatures and certain chemicals. In some applications, it is desirable to provide a fiber-in-ferrule (or capillary-in-ferrule) that may be immersed within solutions of extreme pH, solvents that degrade adhesives, vacuum or elevated pressure and temperature environs. In many, more routine applications it is desirable to provide a seal to prevent migration of gasses or other contamination from the environment into a package intended to isolate an active device, e.g., a semiconductor laser. A ferrule may also act to redirect overfill energy in fibers used in a high power coupling application such as a industrial welding and medical laser delivery systems. Where a fiber must pass through a bulkhead, e.g., from a monitor to within a chemical reaction process stream, chemical and thermal resistance may be critical in maintaining the system seal.
In all cases, it is important to minimize stress induced by thermal expansion mismatches between the ferrule, the carried optical fiber or capillary element and any other materials (polymer adhesives, solders, etc.). The stresses generated by thermal expansion mismatch can degrade optical and mechanical performance of the overall package. By directly fusing fiber or capillary within ferrules made of like material, thermal expansion mismatch stresses are eliminated. Residual physical stresses and strains do remain in traditional packages, however, due to the extremely low contact angles in the fusion region. As is generally known in the scientific glassblowing community, low contact angles in welds are extremely susceptible to failure due to these residual stress concentrations. While post-fusion annealing can reduce the stresses in the weld, considerable stress generally remains and, in the case of fiber and capillary terminations, true post-fusion annealing is rarely possible due to the local presence of thermally labile materials (fiber and capillary polymer coatings, strain relief materials, etc.).
It has been known in the art to seal fibers to ferrules by several methods including adhesives, soldering of metallized fibers and ferrules or by swaging material around the fiber. Many attempts have been made to seal terminations by fusing a fiber together with the ferrule. While generally successful in cases where fiber or capillaries are not doped or employ dopants that do not radically alter thermal properties of the glass or easily diffuse throughout the glass at elevated temperatures, attempts with doped core fiber such as that used in telecom/datacom applications and polymer clad fiber have had limited success. Seals generally fail to hold, or the fiber and/or the ferrule fails, particularly when fiber and ferrule are of differing materials, coupling efficiencies are poor where core dopants diffuse under heating and where polymer clad fibers—necessarily bare of polymer near the fusion, leak into the ferrule at points of contact therein. Where each is made substantially of similar glasses, fusing tends to distort tiny telecom fiber cores to such an extent that the fiber's optical characteristics are changed. This distortion is primarily due to asymmetric, low contact angle stresses in the fusion seal and generally lengthy and poorly reproducible fusion regions. In the case of single mode or polarization maintaining fibers, the fiber geometry is crucial to its proper operation. It is extremely difficult to maintain uniformity of stresses and dimensions during a fusing operation and, due to the small diameter of both core and cladding (and in the case of polarization maintaining fibers, the precise stress distributions within the core composite structures), such difficulty renders the practice of fusion terminations for these type fibers impractical.
Even in cases where fusion has proven viable, e.g., so-called “Specialty Fiber” (fiber for power and spectrum transmission as opposed to data) terminated within quartz and silica ferrules, the residual stress within the structure of the fiber and the ferrule, which may lead to premature failure. Moreover, in the case of providing a preferentially softenable material as an interface between a telecom-type fiber and ferrule, asymmetric stresses due to low contact angles and non-centrosymmetric, extended fusion regions remains and the potential for premature failure remains.
Finally, for lower cost, higher Numerical Aperture (NA) specialty fiber where polymer claddings are employed instead of fluorine doped silica claddings, contact between the bare glass core and the ferrule, in both fused and non-fused regions, must be kept to a reproducible minimum. While the input face may be reproducibly located in space for reproducible fusion thickness (where the nascent core dimension is lost in the fusion to the larger glass ferrule) as the fiber in free space is clad efficiently by air, additional points of contact between the bare core and glass are sites of light leakage and must be avoided. A competing requirement is that the space between the glass ferrule bore and the fiber core must be kept to a minimum in order to maintain concentricity and axial alignment of the fiber within the ferrule. The invention disclosed herein addresses these conflicting requirements, facilitating a low cost, high energy and hermetic termination for polymer clad silica fiber.